Semiconductor devices such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) or IGBTs (Insulated Gate Bipolar Transistors) have been widely used for household electrical appliances, communication apparatuses, power conversion apparatuses or power control apparatuses such as for vehicle-mounted motors, and the like. For the semiconductor devices, a high-speed switching characteristic or a reverse blocking characteristic (breakdown voltage) of several tens to several hundreds of volts is required in many cases.
The on-resistance of the semiconductor devices greatly depend on the electrical resistance of a drift region. The electrical resistance of the drift region depends on the concentration of impurity in the drift region. The concentration limit of impurity in the drift region is determined according to the breakdown voltage of a p-n junction formed between a base region and the drift region. That is, the breakdown voltage decreases when the concentration of impurity in the drift region is increased, while the concentration of impurity in the drift region is lowered when the breakdown voltage is increased. For this reason, there is a trade-off relationship between the breakdown voltage and the on-resistance.
As one means of decreasing the on-resistance while maintaining the breakdown voltage, there is a method in which a super junction structure is used for the drift region. In the super junction structure, a plurality of p-type pillar regions and a plurality of n-type pillar regions are alternately provided in a substrate in-plane direction. In the super junction structure, by making the amount of impurity contained in the p-type pillar region equal to the amount of impurity contained in the n-type pillar region, the concentration of impurity in the drift region can be increased while maintaining the breakdown voltage.
In the semiconductor devices, however, a technique for further improving the breakdown voltage while suppressing an increase in on-resistance is required.